Fabricated nanoparticles are playing an increasingly important role in many technological applications. Representative fields where designed nanomaterials yield improved performance when compared to bulk materials or micron-scale materials include energy and fuel engineering where nanoparticles have use in fuel combustion catalysts, as substrates for solar energy conversion and other applications. Similarly nanoparticles have multiple uses in health care including use in therapeutics, drug delivery devices, and as detection modalities for diagnostics. Nanoparticles may also be used in computing, for example, as transistors and other electronic devices or as storage media, as structural materials for improved strength or tensile properties, and in a host of other applications.
In certain cases, it is the composition of nanoparticles that determine particle performance or the properties of interest. In other cases, the size of a nanoparticle or the surface area, or the ratio of surface atoms to bulk atoms dictates particle performance characteristics. Likewise, in certain applications it is the shape of nanoparticles that dictates observed properties. In many instances, it is a combination of factors. For example, in surface enhanced Raman scattering (SERS), molecules positioned in close proximity to nanometer-scale noble metal nanoparticles exhibit large increases in scattering intensity owing to increases in the electromagnetic fields at the particle surface. SERS has been extraordinarily well-studied from both the experimental and theoretical perspective, and it is now well understood what factors control or influence the behavior of SERS-active materials. In particular, with respect to SERS applications, nanoparticle composition, size and shape are all important factors in predicting the enhancement at a given excitation wavelength.
It is possible to synthesize single nanoparticles. It is also possible to make large numbers in parallel. For the study of fundamental properties, small numbers of nanoparticles may be sufficient, and studies of the properties of single nanoparticles abound. For commercial applications, generally larger numbers of particles may be necessary. For example, if 107 nanoparticles are needed for each dose of a nanoparticulate drug, and 108 doses are desired, a total of 1015 nanoparticles are desired. While the quantity requirements for particles for health care may appear large, it is relatively small compared to materials science or engineering applications where tons of material may be needed.
Nanoparticles can be fabricated in the solid phase, in the liquid phase, and in the gas phase. Depending on how the nanoparticles are made, certain shapes of particles tend to be preferred. The most common shape of nanoparticle is spherical or semispherical. Nanoscale spherical objects are actually faceted nanocrystals that closely resemble spheres. Other common shapes include plates and tubes, for example carbon nanotubes. Recently, it has become possible to fabricate nanoparticles of a variety of more complex, anisotropic shapes, including cubes, prisms, and even tetrapods. Likewise, methods have been described that yield core-shell particles, hollow particles, and onion-like particles within particles. Presently, most of these shapes can only be fabricated through solution-based methods, which typically offer greater flexibility with respect to available processing steps than solid phase or gas phase fabrication methods.
Various methods of nanoparticle growth have been studied, and often can be described in the terms used for the fabrication of polymeric materials: initiation, growth, termination. A first approach to solution based nanoparticle synthesis employs these steps. Initiation involves formation of a seed particle, which grows in size until depletion of material for growth leads to termination. The shape of the seed can often dictate the final shape of the nanoparticle. For example, spherical seeds most often lead to spherical final nanoparticles. In some cases, though, spherical seeds can lead to highly anisotropic particles, because of widely differential growth rates on certain crystal faces of a material. The differential growth method has been used with great success in forming semiconductor particles and metallic nanoparticles with non-spherical shapes.
Despite the general advantages of solution-based nanoparticle synthesis methods over gas phase and solid phase methods, and despite the proven successes in making non-spherical nanoparticles, typical solution fabrication methods are severely limited regarding the possibility of designing non-typical nanoparticle shapes. For example, only high symmetry species can be prepared, meaning that only a small fraction of shape space can be sampled. Therefore, the ability to make nanoparticles in the shape of for example, an open tube, or an arrow, or a spiral, or a hook, a letter of the alphabet, or other complex shapes, is well beyond the capabilities of known solution-based syntheses methods.
Such complex shapes would be highly interesting from a fundamental perspective, and more importantly, could have significant performance benefits. For example, a particle in the shape of a test-tube might be excellent for the controlled release of molecules. Arrow-shaped particles might exhibit superior flow properties, and might exhibit improved penetration into skin compared to spherical particles in uses such as gene delivery. Hook-shaped particles may provide adherence via physical methods, potentially eliminating the need for van der Waals, ionic, or covalent chemical bonding or attachment interactions. Nanoparticles with alphabet shapes might be used as nanoscale taggants for brand security and/or anti-counterfeiting applications. In SERS applications, there are a variety of shapes that might generate unprecedented enhancements. High performance shapes might be even more complex than hooks, arrows, or spirals.
A second approach to nanoparticle synthesis in solution features the use of templates. Templates dictate the shape of synthesized nanoparticles by confining particle growth to the dimensions defined by the inside the template. For example, semiconductor nanoparticles can be made by precursor precipitation within fused vesicles. In such embodiments, the nanoparticle is constrained in size by the size of the vesicle, which in turn dictates the amount of precursor. Furthermore, the particles generally adopt the shape of the vesicle, which is typically spherical. A second example involves striped metallic nanorods, which may be prepared by sequential electrochemical deposition in to cylindrical pores. The particle diameter is defined by the pore diameter, and the particle length is defined by the number of stripes, and the length of each stripe. In principle, templates can be created with pores of different shapes, causing different shapes of particles to be formed. For example, a simple variation of a cylindrical pore is a toothpick-shaped pore, i.e. a cylinder with a reduced diameter at one end, and an expanded diameter at the other.
Current template-based methods of nanoparticle formation are also extremely limited in several respects. In the case of electrochemically-generated striped metallic nanowires, the growth method only works for conductive materials. Much more importantly, the nature of typical template synthesis is inherently two-dimensional. Synthesis is only occurring within the pores of the template, which is essentially a plane of growth situated in a three-dimensional solution. Thus, the number of particles fabricated per unit time is not defined by the volume of solution but rather by the number of pores in the template, typically 108-1010 pores per square cm. This compares poorly with conventional solution-based nanoparticle syntheses, where 1011-1015 particles can be made simultaneously in solution, leveraging the inherently three dimensional nature of a solution.
Yet another significant weakness of known template synthesis methods is the inefficient use of the template itself. In the case of striped metal nanoparticles, the particles are freed by template destruction (via dissolution), so only one particle is synthesized per pore. In certain instances, where the particle length is small compared to the pore length, many particles may be synthesized in a pore, but even then, the template is destroyed to release the particles. In the case of vesicle or micellar synthesis, the particles are isolated with a lipid coating, which is then removed. In another example of template-directed particle synthesis, a typically spherical particle core is used as a template, and a shell particle is grown over it. If the core particle is consumed in the shell synthesis, a hollow particle ensues; when it is not, the core remains inside the shell. In both cases, the template can not be re-used. In short, known template-directed methods are stoichiometric, and do not provide the opportunity for template re-use. In the specific case of the synthesis of striped metallic nanowires, this is not problematic, as the templates are readily available and are relatively inexpensive. However, similar templates having pores defining novel shapes such as arrows, spirals, hooks, or alphabet letters would require extraordinary means (such as LIGA or nanoimprint lithography or e-beam lithography or MEMS or NEMS) to fabricate. These are inherently two dimensional methods that can generate only relatively small numbers of templates, at high cost, making synthesis impractical to accomplish in a stoichiometric manner.
Thus, while known templates offer a method to make particles that might not be possible by the seed-based growth of particles in solution, these techniques are limited both by the inefficient use of the template material, and in many cases, further limited by the two-dimensional nature of the growth process. For unusual shapes, a further and substantial limitation is the cost of template fabrication. It is therefore not possible by any known method to make large numbers of nanoparticles of selected complex shapes.
The present invention is directed toward overcoming one or more of the problems discussed above.